The present invention relates to electric transformers, especially distribution transformers and the protective equipment therefor. More particularly, the invention relates to apparatus for protecting low-voltage distribution transformers from damage due to current surges entering the secondary windings of the transformer. Still more particularly, the invention relates to a low-voltage, two-stage gapped surge arrester.
Distribution transformers convert electrical energy from primary, high voltage levels, such as 2.4 to 34.5 KV, to secondary, low voltage levels, low voltage being defined as 1200 volts and less. Most typically, secondary side voltage levels of distribution transformers are 120/240 volts or 240/480 volts. Distribution transformers include primary and secondary windings which are enclosed in a protective metallic housing. A dual secondary voltage, such as 120/240 volts, is achieved by constructing the transformer secondary winding in two halves or sections. One end of each of the two winding sections is electrically joined to the other and typically grounded at this point of interconnection. In this configuration, when the transformer is energized, the voltage between the grounded interconnection point and each ungrounded winding end will be the same, i.e., 120 volts, and will be equal to one half the voltage between the two ungrounded ends, i.e., 240 volts.
Connection to the transformer windings is made at the transformer terminals, termed bushings. Bushings for both the primary and secondary terminals are usually of the stud type, the stud comprising a conducting bar or rod electrically connected to the internal windings and extending through the protective metallic housing. An insulating material surrounds the stud where it passes through the housing to insulate the stud from the housing. The studs have provisions for the mechanical connection to line wires. Bushings are usually located on the side of the transformer housing, except that primary bushings for 7.2 KV and higher service are usually mounted on the cover or top of the housing. The primary or high voltage terminals of distribution transformers are conventionally designated as the H.sub.1 and H.sub.2 bushings. The low voltage or secondary side line-potential terminals are designated as X1 and X3, while the low voltage neutral bushing is designated as X2.
The majority of distribution transformers are designed for pole mounting; however, some are built for pad or platform mounting. Distribution transformers of both types are susceptible to damage from lightning induced surges entering their windings. When a lightning surge occurs, the voltage appearing across the primary winding may exceed the insulation strength of the winding, resulting in a flash-over across or through the winding insulation, thereby causing the transformer to fail. It has been conventional practice to provide lightning protection for distribution transformers by means of lightning arresters applied to the primary, high voltage winding. More specifically, in the case of single phase distribution transformers in which both primary bushings H.sub.1 and H.sub.2 are at line potential, lightning arresters have typically been connected between H.sub.1 and ground and between H.sub.2 and ground. In applications in which primary bushing H.sub.1 is at line potential and H.sub.2 is grounded, it is common to connect a single lightning arrester between H.sub.1 and grounded H.sub.2. The lightning arrester's function is to provide a path by which lightning induced current readily finds its way to ground without flashing over insulation of the transformer's winding.
Investigations have been made in recent years concerning lightning induced failures of common designs of overhead and pad mounted distribution transformers. These investigations revealed that despite the presence of state-of-the-art primary-side lightning protection as described above, many such transformer failures are attributable to lightning surges entering the transformer via the normally unprotected low voltage terminals, causing failure of the high voltage winding due to the induced voltages. While lightning induced currents entering the low voltage bushings are normally non-destructive, current surges over 5,000 amps are not uncommon. Secondary surges in the order of 3,000 amps can result in potentially destructive induced voltages in the primary winding which may cause the transformer to fail. Thus, it has been determined that primary side arrester protection of the high voltage winding is ineffective in preventing transformer damage caused by secondary side current injection.
Surge currents can enter the low-voltage or secondary terminals of a distribution transformer in three basic ways. The first and most obvious way is due to direct lightning strikes on secondary service conductors. In this case, surge currents are forced through the transformer secondary windings on their way to ground at the transformer neutral X2. This mode of current surge may involve only one half, or the entire secondary winding.
A second possible mode of surge current injection into the low voltage windings of a distribution transformer is due to lightning discharge into the ground near a secondary service point. Such a discharge can cause a local elevation of ground potential resulting in ground currents flowing outward from the discharge point back toward the transformer's grounded neutral X2. Some of this current can flow through the transformer secondary windings via the grounded transformer neutral resulting in a low-side current surge.
The third way that current surges enter low-voltage windings may be less obvious than the others, but is perhaps the most common in occurrence. Lightning strikes to overhead primary-side phase conductors are conducted to ground at the service pole supporting the transformer by a ground wire running down the pole. The lightning arrester connected to the primary winding of the distribution transformer forms one path for the surge from the phase conductor to flow to this ground connection. Where there is an overhead neutral conductor, it is connected directly to the pole ground and it too will conduct surge currents through the ground wire. Since the transformer neutral X2 is also connected to this ground wire, part of the current discharged on the primary side can be diverted into the secondary winding of the transformer.
In each of the last two cases, surge current may enter the grounded neutral terminal X2 of the low-voltage winding and divide through the two halves of the winding, exiting by way of secondary line terminals X1 or X3, or both. For such current to flow through the transformer, there must be a path through the customer load or customer meter gaps, or across gaps in the customer's wiring. Where such a path exists, the amount of surge current conducted through the transformer secondary windings will be dependent both on the amount of customer load connected at the time of the surge and, more significantly, on the ratio of the resistance of the pole ground to the resistance of the customer ground. If the pole ground has a resistance less than that of the customer ground, the current level within the transformer should be well below that required to produce an insulation failure within the windings.
Three-wire surge injection occurs where a surge enters the transformer through X2 and departs from the transformer through both X1 and X3. Two-wire surge injection occurs in two situations. First, it may occur when the surge enters the transformer through X2 and departs from the transformer through either X1 or X3. This can occur when only one customer meter gap fires, or when the load on the service conductor connected to X1 is substantially different from that on X3. Two-wire injection may also occur when a surge enters either X1 or X3 and exits to ground through X2.
Depending upon their design, distribution transformers tend to be particularly affected by certain types of surges. More specifically, transformers having uncompensated winding constructions, i.e., noninterlaced low voltage windings, are particularly affected by both three-wire and two-wire surge injection. Transformers having compensated winding constructions, i.e., interlaced low voltage windings, are only affected by two-wire surge injection. The majority of modern day distribution transformers have noninterlaced low voltage windings, and thus are particularly susceptible to damage from both three-wire and two-wire surge injection.
In an effort to protect distribution transformers from such secondary-side surges, various schemes have been employed. First, constructing the transformers with interlaced secondary windings provides good protection from three-wire surges, but, as explained above, two of the most common types of secondary surges result in two-wire surge injection and interlaced windings offer no protection from such surges. Further, transformers having interlaced windings also are more expensive than those with noninterlaced windings.
Alternatively, or additionally, extra primary winding insulation may be added to provide some protection from both two and three-wire surge injection. This technique is relatively expensive, however, and does not prevent surges from entering the transformer, but merely serves to raise the damage threshold level of the transformer.
Recently, surge arresters of the metal oxide varister (MOV) type have been applied between secondary-side phase terminals, X1 and X3, and the grounded neutral terminal, X2. This has been shown to provide adequate two and three-wire surge protection, but the MOV arresters are expensive and, since they are typically mounted within the transformer housing, they are not visible to workers. Thus, workers cannot determine from a simple visual inspection whether the transformer is protected by secondary side MOV arresters or whether the arresters are still functional. Furthermore, the MOV arresters have relatively less clamping action for high energy surges due to the voltage drop across the impedance of their interconnecting leads.
Accordingly, there remains a need in the art for a low voltage surge arrester capable of protecting a distribution transformer from damage or destruction caused by surge currents entering the secondary windings. Preferably, such an arrester would be effective against both two and three-wire surge injection, and would be reliable and inexpensive and provide tight clamping of high energy surges. One means by which this end may be accomplished is to design a low voltage arrester employing spark gap technology.